Alphanumeric ink droplet recorder



D. E. DAMOUT H ALPHANUMERIC INK 'DROPLET RECORDER I May 12, 1970 s Sheets-Shet 1 'Fiied Dec. 2s, 1967 PRESS- O INTELL. 0 SOURCE INK RESERVOIR FIG.

INVENTOR. DAVlD E. DAMOUT A T TORNE Y5 May 12, 1970 D. E. DAMOUTH 3,512,173

ALPHANUMERIC INK DROPLET RECORDER Filed Dec. 28, 1967 3 Sheets-Sheet 5 llllllllllll 56 l'm m INVENT OR.

DAVID E. DAMOUT BY M ATTORNEYS United States Patent 3 512 173 ALPHANUMERIC 11 1K DROPLET RECORDER David E. Damouth, Rochester, N.Y., assignor to Xerox Corporation, Rochester, N.Y., a corporation of New York Filed Dec. 28, 1967, Ser. No. 694,264

Int. Cl. G01d /18 Int. Cl. 346-75 6 Claims ABSTRACT OF THE DISCLOSURE This invention relates to recording apparatus generally, and, more specifically to apparatus for recording alphanumeric symbols or characters by precise deposition of discrete charged marking droplets on a recording medium.

The ever present desire for high speed alphanumeric recorders has been fulfilled by prior art ink droplet recorders. However, since this capability has matured into practical application in the field of recording, much design work has concentrated on improving the quality of the recorded characters. Little success has been met in overcoming a basic defect in the quality of the characters recorded in this manner. This defect is very apparent in the generally vertical segments of the characters which normally are represented by a solid straight line. However,

because of the drop-by-drop deposition employed in prior art systems these segments appear as a series of dots aligned vertically. Since most alphanumeric symbols or characters include such vertical segments, this dotted or broken appearance detracts from the overall quality of the recording.

Therefore, it is an object of the present invention to improve the quality of alphanumeric symbols recorded by the deposition of charged marking droplets on a recording medium.

It is another object of the present invention to provide an improved ink droplet recording system.

These and other objects which may become apparent are accomplished in accordance with the principles of the present invention wherein discrete droplets of ink are charged electrostatically to an extent determined by their desired position in an area of a recording medium to form a particular alphanumeric symbol. The charge of each droplet influences the amount by which each is deflected by a pair of deflection plates as it passes toward the recording surface. The amount of charge imparted to each droplet is a function of the amplitude of that portion of a ramp or sawtooth signal which is gated in accordance with the particular character to a charging electrode.

For a better understanding ofthe invention as well as other objects and features thereof, reference may be made to the following description of the invention to be read in connection with the accompanying drawing wherein:

FIG. 1 represents the recording apparatus in accordance with the principles of the present invention;

FIG. 2 is a block diagram of the circuit used in conjunction with the apparatus of FIG. 1;

FIG. 3A and B represent the layout of the recorded characters;

FIG. 4A, B, C, and -D represent waveforms generated by the circuit of FIG. 2; and,

3,512,173 Patented May 12, 1970 FIG. 5 illustrates an optional compensating circuit which may be utilized.

Referring now to FIG. 1, the apparatus includes a conduit 10 connected at one end to an ink reservoir 11 and terminating at its other end in a nozzle 12 having a mouth or orifice 14 through which ink is ejected. Ink 15 is forced into the nozzle under pressure generated from a pressure source 13 which exerts pressures generally of at least 10 psi. and above. The preferred pressures will vary depending largely upon the desired speed of printing and the size and shape of orifice 14. The nozzle 12 as shown is of generally conical cross-section converging to orifice 14 which may be circular, and, for example, may be about 0.0015 inch in diameter. Both the nozzle and the mouth thereof may be of other suitable symmetric or asymmetric design.

Under normal operating conditions, the ink forced into the nozzle 12 is ejected from the orifice 14 under pressure generated from source 13. The pressure, in the absence of controls to be described, is adequate to cause the ink to be ejected in a continuous stream. Due to the natural instability of a liquid stream, the stream inherently breaks up into droplets a short distance from the nozzle. The inherent breakup process is of a random nature in both rate and size of the droplets.

It has been found that by applying controls to the stream the droplet breakup becomes very regular, and surprisingly uniformly shaped droplets are formed. To achieve this result in accordance herewith, nozzle 12 is vibrated at an ultrasonic rate which in the preferred embodiment is at a frequency vibration on the order of approximately 30 kc. However, other frequencies may also be utilized depending on the particular size and spacing of the droplets desired. Vibration may be effected by means of a piezoelectric crystal 17 positioned directly about the converging end of the nozzle. In a preferred embodiment, as illustrated, a mechanical structure 18, comprising a rigid plate member or the like, snugly surrounds the nozzle end. A piezoelectric Bimorph bender crystal 17, supported by and extending between anchors 19- and 20, is directly connected at approximately right angles to plate 18. An electrical signal as on the order of 10 volts and at a frequency desired, is applied from a potential source 16 to vibrate the crystal, which in turn relates a mechanical stress at the same frequency rate to the plate member 18. Being connected directly to the nozzle, plate 18 causes vibration thereof at the transmitted frequency which is effective in uniformly controlling size and breakup rate of the ejecting ink stream to produce the uniform droplets 22.

Surrounding the stream at the vicinity where the droplets break off is a charging electrode 23 which may be cylindrical or other suitable geometric configuration. Information in the form of intelligence signals from a source 25 applies a potential to the electrode 23 creating an electric field between the electrode 23 and the electrically conductive droplet ink stream at the position of droplet formation. The field generated thereby induces a charge on the surface of the continuous ink stream. The charge on any portion of the ink stream surface is proportional to the electric field present at that surface which in turn is proportional to the voltage applied to electrode 23. In particular, as a droplet breaks away from the end of the stream, the charge on its surface is proportional to the voltage applied to electrode 23. After the droplet has separated, its charge can no longer change, since it is now electrically insulated by the surrounding air. Thus, the charge on the individual droplet is nearly proportional to the voltage applied to electrode 23 at the time the droplet breaks from the stream.

There is however a small error introduced as follows: As each droplet is formed, the electric field at its surface is not due entirely to the charging electrode 23 but will be affected by any charge present on the droplets most recently formed, since these are still very close to the droplet being formed. This results in an overshoot, such that when the charging electrode 23 is pulsed for a time long enough for at least several droplets to form, the .first droplet will have slightly more. charge than subsequent droplets. V

This effect can be compensated for by a simple electrical network included in intelligence source 25- which will be described hereinafter in connection with FIGS. 2, 3, 4, and 5.

In its simplest form, the voltage applied to the charging electrode 23 will be a DC voltage. which may be turned off or on or varied in amplitude in accordance with intelligence signals emanating from information source 25, which may be a pulse generating circuit for responding to character codes to record alphanumeric information. The details of a possible form that this circuit may take will be described hereinafter. Thus, each droplet after passing electrode 23 will either be charged or uncharged, depending on the voltage generated by the intelligence signal when that droplet separated from the stream.

It should be understood that the term uncharged as used in the preceding paragraph means not charged in accordance with intelligence signals from source 25. However, as will be seen hereinafter in connection with undesired droplets, all droplets emanating from the output side of charging electrode 23 will have a certain charge. Generally a steady state is maintained on electrode 23 by applying a steady DC. potential thereto and a signal potential of controlled amplitude and width superimposed thereon. By this means, each droplet will bear charge.

Those droplets bearing a charge representative of information to be recorded are to be deposited onto a print receiving medium 40 while the other droplets are retrieved and returned to the system. Accordingly, after passing electrode 23, each of the ink droplets follow .their trajectory through a pair of divergent electrode plates 27 and 28 between which is applied a high voltage from a potential source 29. The field generated between the latter electrodes is effective to deviate or defleet from the trajectory either those droplets which have or have not been charged as a function of the appropriate intelligence pulse from source 25. Thus, as illustrated, the latter droplets are deflected by means of the existing field toward electrode 28 into an ink draining channel 30 formed by an interception plate 31, and a flared diverging lower extension 32 of electrode 28.

The droplets which have been charged as a function of the appropriate intelligence pulse from source 25 are deflected toward electrode plate 27 to an extent dependent upon the charge they bear and onto the recording medium 40. This recording medium is moving out of the plane .of the drawing in FIG. 1 toward the viewer at a rate proportional to the, rate at which the intelligence signals are applied to charging electrode 23.

' As can be appreciated, the imposed intelligence signal must be accurately produced and transmitted so that each printing droplet strikes the paper in a position on the surface thereof which is a function of the output potential of source 29 and the charge on the printing drop- .let. The charged state of those droplets not subject to the signal pulse is not critical so long as it is sufficient to cause the droplets to be intercepted by channel 30 under the influence of the field applied between plates 27 and 28.-Those ink droplets intercepted by channel 30 are then funneled via a hose connection 35 to an ink reservoir 36. A pump 37 operated by a float control (not shown), for example, in reservoir 36 returns the surplus ink via conduit 3-8 and through check valve 39 to main reservoir 11.

In a typical embodiment a representative signal DC potential applied at 25 is in the order of 200-400 volts.

This is dependent on various parameters including nozzle characteristics, droplet direction relative to copy surface 40, the velocity of the ink stream, size of droplets, magnitude of deflection potential 29, etc. A typical ink stream velocity would be-30 ft./sec. and above produced with a nozzle orifice about .0015" in diameter and 30 pounds per square inch ink pressure. In this manner, it is possible with approximately 3000 volts applied between electrodes 27 and 28 to selectively divert individual droplets from the stream. The response time of the system is essentially equal to the reciprocal of the drop production rate, i.e., 33 ms. for a 30 kc. drop production rate.

Reference will now be made to FIGS. 2, 3, 4, and 5 in describing the novel features of the intelligence source 25 hereinabove referred to in connection with FIG. 1

As shown in FIG. 3, an alphanumeric symbol or character may be printed utilizing the apparatus of FIG. 1 when the character pattern to be recorded is sub-divided into a 5 x 7 matrix, for example; For the character F, FIG. 3A illustrates those sub-divisions of this matrix which are represented by a pulse to the charging electrode 23 in order to record this character. The ones in this matrix are representative of the presence of an appropriate pulse while the zerosare indicative of the absence of a pulse. i

As will be seen in the description of the circuit of FIG. 2 and the waveforms of FIG. 4, these pulses are generated in a particular sequence corresponding to the numerical order shown in FIG. 3B where the numerals 1 through 35 represent a possible thirty-five pulses'which may be applied to the charging electrode 23 in succession. The characteristics of the pulses so applied and the arrangement of the electrode plates 27 and 28 are such that if the entire thirty-five pulses were applied to the charging electrode five substantially parallel vertical lines would be recorded on the recording medium 40 in the form of contiguous dots of ink droplets. By selectively eliminating predetermined pulses in this sequence (such as those corresponding to the sub-divisions in the matrix of FIG. 3A represented by a zero for the character F) a character can be generated from the format of these parallel vertical lines.

Referring now to FIG. 2, which shows the circuit which comprises intelligence source 25 in FIG. 1, and FIG. 4, which illustrates the waveforms generated within this circuit, there is illustrated a conventional input/ output interface circuit 42. This interface circuit may contain proper storage for digital code representations of a desired character to be recorded and may interface with a variety of input sources such as a conventional keyboard, punch tape reader, telephone line, or other source of digital signals. The'output of the interface unit provides a parallel input to a conventional decoder circuit which decodes the digital code representation and generates a character pulse on one of a series of parallel outputs indicative of the code character received by the input/output interface 42.

A character matrix 46 receives the character pulse from the decoder circuit 44 to generate in cooperation with a group of parallel AND gates, represented generally by the block 48, a sequence of preferably uniform amplitude pulses representative of-the coded character.

The character matrix 46 may take various forms and, as an example only, mayvconsist of a conventional diode matrix having a number of character select input wires which correspond to the outputs of the decoder circuit 44. These wires are selectively coupled to read-out wires via diodes which are forward biased when their respective character select wire is energized. In the case of the character F, fourteen diodes are individually connected between the appropriate characterselect wire "and to the read-out wires corresponding to subdivisions 1 through 7,

11, 14, 18, 21, 25, 28, and 35. Each read-out wire provides one input to an AND gate in block 48. In the case of a x 7 matrix there would be thirty-five AND gates in block '48.

Each of the AND gates in block 48 has two inputs, one from the particular read-out wire associated with it and another from a particular register in the conventional forty bit counter 50. This counter is driven by clock pulses from a conventional clock source 52. These pulses are shown in FIG. 4A as clock pulses 56. The sequence pulses in this serial stream is determined by the sequence of diodes coupling the particular character select wire to one of the thirty-five read-out wires.

As shown in FIG. 4 forty clock pulses can be efliciently used to generate one character pulse pattern during one recording cycle. While each recorded character consists of a predetermined pattern of sub-divisions of the matrix shown in FIG. 3A where a possible 35 sub-divisions exist, five additional stages are used in the forty bit counter 50. These permit allowance for a retrace time between successive vertical segments of the character to be recorded.

As will be seen in the description of the circuit of FIG. 2, a sawtooth generator 64 is utilized which necessitates a period at the end of the ramp portion of the sawtooth during which its amplitude returns to an initial level, this period being referred to commonly as a retrace interval. These stages corresponding to the retrace interval and spaced every seven stages in the bit counter 50 may be utilized to space sequences of seven pulses to the charging electrode 23. These stages are utilized by merely providing a stage in the counter 50 to register these time spaces without effecting any other portion of the circuit. In this manner, as the first seven pulses are counted by bit counter 50, seven count pulses are provided to a sequence of seven AND gates in block 48. Thereafter, the eighth count pulse is generated upon the receipt from clock 52 of the eighth clock pulse while no output pulses are generated from counter 50 to the AND gates 48. Upon receipt of the ninth clock pulse, the bit counter 50 generates another count pulse to the appropriate AND gate in block 48 and the next vertical segment of the character will be recorded.

The outputs of the thirty-five AND gates represented by block 48 are coupled in parallel to the input of an OR gate arrangement 58 Which provides a serial stream of uniform amplitude pulses in a particular pattern corresponding to the coded character received at the interface 42. It is understood that the output pulse from any stage of counter 50 is present at that stage until the next clock pulse arrives. Therefore, the duration of the pulse is equal to the time between clock pulses 56.

These pulses are appropriately shaped and amplified by conventional amplifier 60 and supplied as one input to a conventional linear AND gate represented by block 62. The input of this AND gate is coupled to a conventional sawtooth generator 64 which generates a waveform represented by FIG. 4C. Each cycle of operation for the sawtooth generator, i.e., each sawtooth, is initiated upon the counting by conventional eight bit counter 66 of eight clock pulses from clock source 52. It should be noted, however, that this eight bit counter 66 may be so connected with the interface 42 so as to preset it to a seven count before the first character code is processed. There are other ways in which to arrange the circuit of FIG. 2 in order to accomplish the generation of the first sawtooth coincidentally with receipt and processing of the first character code which are well known in the art.

The input waveforms to the linear AND gate 62 for the example character F are represented in FIG. 4B and C. The linear gate will respond to these input waveforms to selectively gate through portions of the sawtooth waveform whichcoincide with the pulses originating at the output of OR gate 58. This out-put from the linear AND gate is represented by the waveform of FIG. 4D. As shown in this FIG. 4D, the waveform consists of a first pulse 66 which actually is composed of seven pulses of equal duration. These are designated by the broken lines in this figure. If the time duration of the ramp portion of the sawtooth is equal to period t then the individual pulses making up this pulse 66 are equal to t/rt where n is equal to seven in the situation of a 5 x 7 matrix. The remaining pulses in the waveform of FIG. 4D have an equal duration of t/ n. It will be noted that all the pulses of FIG. 4D have a linearly varying amplitude. This varying amplitude permits the imparting of differing charges to successive ink droplets passing through the charging electrode 23. The charge on the successive ink droplets will permit the droplets to be very evenly deflected by the electrode plates 27 and 28 so as to form a contiguous vertical line in the case of pulse 66 in the waveform of FIG. 4D.

If the amplitudes of the pulses, for example, making up pulse 66 in waveform of FIG. 4D were of an unchanging amplitude within one pulse then a contiguous vertical line would not be possible since there would be a definite charge disparity between successive droplets or groups of successive droplets which would be deflected in equal amount resulting in the deposition of one or more droplets in a discrete position on a recording medium 40 lending a dotted appearance to any vertical line or segment thereof.

Referring again to FIG. 2 another conventional amplifier 68 is shown having its input coupled to the linear AND gate 62 to amplify the waveform of FIG. 4D. The output of this amplifier then passes through compensating network 67 to charging electrode 23.

The compensating network 67 as more fully shown in FIG. 5 includes a resistor 70 connected between terminal 72 and 74. Terminal 72 is coupled to the output of amplifier 68 as shown in FIG. 2 while terminal 74 is coupled to the charging electrode 23 referred to previously in connection with FIG. 1. The side of resistor 70, which is coupled to terminal 74, is connected to a suitable reference potential, for example, ground potential, by way of a series circuit including resistor 76 and capacitor 78. Th specific values of resistors 70 and 76 and capacitor 78 depend on the parameters of the remaining parts of the recording system, such as droplet size, droplet formation rate, the geometry of the charging electrode 23, etc. These values therefore may be determined emperically and will vary with the parameters employed.

The compensating circuit shown in FIG. 5 will attenuate the initial portion of the signals applied at terminal 72. Therefore, the effect of this compensating network on a pulse, such as pulse 66 in FIG. 4D, is to slightly reduce the highest amplitude portion of this pulse. In other words, this compensating network rounds off the amplitude of the initial portion of any pulse applied at input terminal 72 thereby compensating for the overshoot phenomenon previously mentioned in connection with the description of FIG. 1. The end result will be as described in that the droplets charged during the application of pulse 66 to the charging electrode 23 via the compensating circuit 67 Will each have a charge varying in amplitude in a linear manner between the first droplet entering the influence of the charging electrode to the last droplet entering the influence of the charging electrode during the application of this pulse thereto.

It should be noted that this compensating circuit is not absolutely necessary and may be employed to enhance the copy quality in the final recording.

In summary, therefore, an alphanumeric recording apparatus has been disclosed in which high quality character recording is possible utilizing the concept of imparting different charges to successive ink droplets to permit the deposition of these droplets onto the recording medium in a contiguous manner where the particular character includees vertical line segments as the character is being read.

While the invention has been described with reference to the circuit disclosed herein, it is not confined to the .alent components may be substituted for the components of the preferred circuit without departing from the scope of the invention. Thus, for example, although a diode character matrix 46 has been referred to, it must be realized that other types of character matrices such as magnetic core, resistive, or transistor matrices may also be employed using the concept of this invention. It is also possible to use a completely serial memory, such as a digital delay line, instead of the matrix system shown.

Also, a 5 x 7 matrix has been described in connection with the present disclosure as an example only and that the concepts of this invention are equally applicable to any m by n matrix where m and n are integers.

While the present invention has been directed to a particular type of ink droplet forming device, it should be realized that it is equally applicable with any type of ink droplet forming device which utilizes a charging electrode and deflection of the charged ink droplets.

While the invention has been described with reference to its pureferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements without departing from the true spirit and scope of the invention. In addition, many modifications may be made to adapt a particular situation to the teaching of the invention without departing from its essential teaching.

' What is claimed is:

1. A recording apparatus for recording alphanumeric characters composed of combinations of predetermined -selected portions of an m by n matrix, where m and n are integers, comprising:

(a) an ink reservoir;

(b) nozzle dispensing means for dispensing ink from said reservoir towards a recording medium;

() pressure maintaining means suflicient for causing said ink to be dispensed at said nozzle to form into a continuous stream of droplets;

(d) means for vibrating said nozzle at a frequency resonant with said droplet formation whereby to cause said stream to break up into uniformly sized droplets evenly spaced;

(e) pulse train generating means for generating in response to a character code m sets of pulses, each set having a possible maximum, of n pulses said pulse train generating means including,

(1) sawtooth generator means for generating successive substantially linear ramp signals, one for each of said sets,

(2) pulse means for generating m groups of a possible maximum of n uniform pulses each, each of said groups corresponding to one of said sets, the number and sequence of said uniform pulses being a function of a particular character,

(3) means for synchronizing the beginning of each of said groups with the generation of one of said linear ramp signals,

(4) linear gate means having an output terminal and responsive to said uniform pulses for generating output signals representative of portions of said ramp signals occurring coincidentally in time with said uniform pulses,

(5) means coupled to said output terminal for charging selected ones of said uniformly sized ink droplets as a function of the magnitude of said output signals,

(f) a pair of deflection plates; and,

' (g) means for applying a direct current voltage to said deflecting plates to deflect said selected ones of said uniformly sized ink droplets in accordance with said output signals.

2. An ink recording apparatus for alpha-numeric symbols comprising in combination:

(a) an ink reservoir;

(b) nozzle dispensing means for dispensing ink from said reservoir;

(0) pressure maintaining means suflicient for causing the ink to be dispensed at said nozzle to form into a continuous stream of droplets;

(d) means for vibrating said nozzle at a frequency resonant with said droplet formation whereby to cause said stream to break up into uniformly sized droplets evenly spaced;

(e) electrode means positioned relative to said uniformly sized droplets for imparting a charge thereto as a function of the instantaneous amplitude of a signal applied to said electrode means;

(f) means for applying a sequence of pulses to said electrode means, the amplitude of any one of said pulses varying in a substantially linear manner and the sequence of said pulses being representative of a particular alphanumeric symbol whereby any one of said successive droplets passing said electrode means during the application of any one of said pulses thereto acquires a charge different from that acquired by an adjacent droplet;

(g) a pair of deflecting plates; and, g

(h) means for applying a direct current voltage to said deflecting plates to deflect the ink droplets in accordance with charges thereon.

3. A recording apparatus for recording alphanumeric characters composed of combinations of predetermined selected portions of an m by n matrix, where m and n are integers, comprising (a) an ink reservoir;

(b) nozzle dispensing means for dispensing ink from said reservoir; I

(c) pressure maintaining means sufiicient for causing said ink being dispensed at said nozzle to form into a continuous stream of droplets;

((1) means for vibrating said nozzle at a frequency resonant with said droplet formation whereby to cause said stream to break up into uniformly sized droplets evenly spaced;

(e) charging electrode means positioned relative to said vibrating means for imparting to selective ones of said uniformly sized droplets a charge as a function of the instantaneous amplitude of a signal applied to said electrode means;

(f) a sawtooth generator means for generating at an output terminal successive substantially linear ramp signals, each ramp signal including a portion which is divisible into n subportions of equal time duration, where the duration of said portion is equal to time period t;

(g) pulse generating means for generating at an output terminal a sequence of pulses as a function of one of said predetermined characters, each of said pulses having a duration substantially equal to t/n;

(h) a linear gate having two input terminals coupled to said output terminals respectively;

(i) means for synchronizing the operation of said sawtooth generator and said pulse generating means;

(j) means for coupling the output of saidlinear gate to said charging electrode means;

I (k) a pair of deflecting plates; and,

(1) means for applying a direct current voltage to said deflecting plates to deflect said selective ones of said uniformly sized ink droplets in accordance with the charge thereon.

4. A recording apparatus for recording alphanumeric characters composed of combinations of predetermined selected portions of an m by 11 matrix, where m and n are integers, on a recording medium, comprising:

(a) an ink reservoir;

( b) nozzle means coupled to said ink reservoir for causing ink from said reservoir to be directed toward said recording medium in the form of evenly spaced apart uniformly sized ink droplets;

(c) electrode means positioned relative to said uniformly sized droplets for imparting a charge thereto as a function of the instantaneous amplitude of a signal applied to said electrode means;

(d) means for applying a sequence of pulses to said electrode means, the amplitude of any one of said pulses varying in a substantially linear manner and the sequence of said pulses being representative of a particular alphanumeric symbol whereby any one of said successive droplets passing said electrode means during the application of any one of said pulses thereto acquires a charge dilferent from that acquired by an adjacent droplet;

(e) a pair of deflecting plates; and,

(f) means for applying a direct current voltage to said deflecting plates to deflect the ink droplets in accordance with charges thereon.

5. Apparatus as defined in claim 4 wherein said means for applying a sequence of pulses to said electrode means includes:

pulse train generating means for generating in response to a character code m sets of pulses, each set having a possible maximum, of n pulses said pulse train generating means comprising,

(1) sawtooth generator means for generating successive substantially linear ramp signals, one for each of said sets,

(2) pulse means for generating m groups of a possible maximum of n uniform pulses each, each of said groups corresponding to one of said sets, the number and sequence of said uniform pulses being a function of a particular character,

(3) means for synchronizing the beginning of each of said groups with the generation of one of said linear ramp signals,

(4) linear gate means having an output terminal and responsive to said uniform pulses for generating output signals representative of portions of said ramp signals occurring coincidentally in time with said uniform pulses, and,

(5) means for coupling said output terminal to said electrode means.

6. Apparatus as defined in claim 4 wheerin said means for applying a sequence of pulses to said electrode means includes:

(a) a sawtooth generator means for generating at an output terminal successive substantially linear ramp signals, each ramp signal including a portion which is divisible into n subportions of equal time duration, where the duration of said portion is equal to time period t;

(b) pulse generating means for generating at an output terminal a sequence of pulses as a function of one of said predetermined characters, each of said pulses having a duration substantially equal to t/n;

(c) a linear gate having two input terminals coupled to said output terminals respectively;

(d) means for synchronizing the operation of said sawtooth generator and said pulse generating means; and,

(e) means for coupling the output of said linear gate to said electrode means.

References Cited UNITED STATES PATENTS 1/1967 Lewis et a1 346-75 OTHER REFERENCES JOSEPH W. HARTARY, Primary Examiner U.S. Cl. X.R. 317-3 

